1. Field of the Invention
This invention relates to a method and apparatus for producing a three-dimensional braid structure, such as a multi-layer braid structure, and to a structure produced by such a method and apparatus.
2. Description of the Prior Art
Braided structures are increasingly being used in industry to provide strong, lightweight and non-metallic components. Particular industries requiring such braided structures are the automobile industry and the aircraft industry. The advantage of a braided structure is that such a structure has good tensile strength in all directions as compared with a woven structure which has a relatively limited tensile strength in directions other than those in the direction of the weft and the warp of the yarns comprising the structure.
In order to fit in with industrial requirements, there is a need to provide braid structures in a complex form, that is to say in a form with a cross-section other than that of a simple rectangle or tube, or a moderate variation therefrom. Typical complex forms which are required are forms having, for example, I, J or C cross-sections. Attempts to form such cross-sections in braiding apparatus have previously not been particularly successful since, at any area where there is a re-entrant portion, the yarns of the braid tend to span the entrance and hence defeat the form being sought after.
In other complex forms of structure which do not have re-entrant portions, such as ones sought to have relatively sharp corners or edges, there is a tendency for the braid as laid to be unduly tensioned over the corner or edge and for the braid to open so that the resultant braided structure does not have a uniform strength throughout.
Braided structures are usually of two forms either flat or circular. From "Braiding and Braiding Machines" by W. A. Douglas which was published in 1964 by Centrex Publishing Company, Eindhoven, we know those created in a flat form may be produced in braiding apparatus having a plurality of serpentine tracks and package carriers of yarn which travel the tracks whereby they follow serpentine paths, interbraiding the yarn dispensed by carriers as they do so. At the ends of the paths the carriers are reversed in their direction.
According to US-A-4312261, a traditional way of forming a multi-layer braided structure consists of stacking multiple layers on top of one another and bonding them together, but such structures have virtually no strength in a direction perpendicular to the layers and are liable to fail due to separation or delamination of the layers.
Referring again to "Braiding and Braiding Machines", a braid of a generally tubular cross-section, e.g. circular, may be produced using braiding apparatus in which serpentine tracks are defined in a closed ring and the braid is formed in an area of access of the ring. The yarn package carriers traverse round the serpentine tracks of the ring to follow serpentine paths and lay down the tubular braid as it progresses through the apparatus.
The braid may be formed over a mandrel and this may be of a cross-section other than circular to a limited degree. Multilayer braided structures have been proposed where radial yarns project from a mandrel and the package carriers of yarn weave their yarn around the radial yarns. Such structures have been difficult to manufacture. A novel and improved method and apparatus for constructing a multilayer braid of flat or hollow form where the various layers are interwoven one with the other during the manufacturing process is described in pending U.S. patent application Ser. No. 501043 dated 29 Mar. 1990 and International Patent Application PCT/GB91/00002. The present invention develops the idea of the multilayer structure described in those patent applications.
One proposal which has been made previously to form complex braid structures is that the structure should be developed as a series of components which are then joined together. As a C structure can effectively be constituted of three simple straight structures which are joined at the comers for example by stitching or enveloping in a woven sleeve, the whole can be impregnated if necessary to make a composite braided structure.
Where mandrels are used to create braided structures and a whole range of structures are required there is a disadvantage that a different type of mandrel is required for each size or variation of shape. This considerably increases tooling and production costs. Hence it is obviously advantageous if the range of mandrels required can be substantially reduced in size or eliminated.
In order to overcome the delamination problem and to increase the strength of the structure in a direction which would be at an angle to a layer of a multi-layer structure, it is proposed in US-A-4312261 that a three -dimensional structure be formed by braiding wherein strands extend at an angle to a plane as well as in that plane. That is achieved by releasably maintaining package carriers of yarn in a matrix to form a carrier plane and providing means which effect movement of the carriers along predetermined paths relative to each other in the carrier plane to intertwine the yarn, the movement being effected by moving selected rows and columns along their length by predetermined distances, one after another so that individual carriers are moved in a sequence of discrete steps in mutually perpendicular directions. That is necessarily a slow process and the apparatus must be complex.